Add --signature flags to CLI enrich command

[NTCoding]

Deliverable: CLI enrich command supports method signatures

Context

During extraction workflow testing, we found that the schema supports signature on DomainOp components, the builder supports it, and the UI displays it—but the CLI enrich command has no flags to add signatures.

Key Decisions

• Parameter format: --parameter "name:type" or --parameter "name:type:description"
• Return type format: --return-type <type>
• Flags are repeatable (like existing --reads, --validates, etc.)

Acceptance Criteria

• Given DomainOp, When --parameter "orderId:string", Then signature.parameters includes {name: "orderId", type: "string"}
• Given DomainOp, When --parameter "orderId:string:The order ID", Then signature.parameters includes full object
• Given DomainOp, When --return-type void, Then signature.returnType is "void"
• Given multiple --parameter flags, When command run, Then all parameters added
• Given invalid parameter format, When command run, Then validation error

Related Code

• packages/riviere-cli/src/commands/builder/enrich.ts
• packages/riviere-cli/src/commands/builder/enrich.spec.ts

Migrated from Task Master #63